Foams are materials used in applications where light weight, flotation, thermal insulation, electrical insulation and cushioning effects are required. Foams made out of polystyrene, polyethylene of different densities and polyurethanes are widely known. However, foams made from these materials are limited in thermal resistance, chemical resistance, weathering resistance and ageing properties in general terms. Furthermore, standard types of such foams are highly flammable, and there is a need for foams in critical applications such as aerospace, mass transport, marine, construction, electrical, such as cable, electronic, microelectronic and semiconductor and chemical such as in the petrochemical industries for foams which also have chemical resistance, weathering resistance and good fire resistance. Foams made from fluoropolymers can fulfil these requirements.
U.S. Pat. No. 5,885,494 (du Pont) discloses a composite tubular structure which comprises a layer of first fluoropolymer and a substrate in contact with said layer, wherein the first fluoropolymer is foamable at a temperature at which the substrate is either less foamable or is non-foamable. The intention of the disclosed method is to prevent foaming of the substrate compared to the first fluoropolymer layer and one way of doing this is to crosslink the substrate material. There is no disclosure however of the first fluoropolymer layer being crosslinked.
U.S. Pat. No. 5,837,173 (Ausimont) relates to a process for preparing expanded articles based on ethylene-chlorotrifluoroethylene copolymers having high dielectric characteristics, in which nitrogen is injected directly into a molten polymer blend in a continuous process to result in expansion of the polymer. No crosslinking of the polymer is disclosed.
The U.S. Pat. No. 4,615,850 and U.S. Pat. No. 4,675,345 describe the use of chemical blowing agents combined with plasticizers and nucleating agents to produce foams of PVDF homopolymers and copolymers and their mixtures. The U.S. Pat. No. 5,883,197 and U.S. Pat. No. 6,506,809 describe the use of specific nucleating additives to achieve better quality foams, while the U.S. Pat. No. 4,425,443 describes the limitations of using certain chemical blowing agents, and in particular the thermal degradation of the resin which can occur. Again no crosslinking of the polymer is disclosed in any of these patents.
The patents EP 223155 and JP 06047857 A describe the use of a solvent, preferentially dichlorodifluoromethane as the expansion agent for fluoropolymer foams. The materials are not crosslinked and these processes have the disadvantage of using costly solvents which must be carefully removed so as not to deteriorate chemical resistance and/or purity.
WO 2005/105907 in the name of the present applicant describes the use of high pressure autoclaves to produce high quality fluoropolymer foams by directly impregnating formed artefacts with inert gas and then reducing the pressure to expand the material. The fluoropolymer resins are crosslinked to allow the impregnation and expansion processes to take place well above the normal softening point of the resin.